How would a Central Office telephone switch instruct a D4 channel bank to ring an analog phone?

A.
The central office telephone switch toggles the A/B bits in the direction of the CPE

B.
The central office telephone switch sends a digital ring pattern in-band on the channel

C.
The central office telephone switch toggles the R bit in the direction of the CPC

D.
The central office telephone switch uses a separate channel reserved just for ringing the phone

Explanation:
For D4 channel banks, a four-state signaling scheme is implemented through the use of an ‘A-bit’ (transmitted in the 6th frame), and a ‘B-bit’ (transmitted in the 12th frame).